In an optical disk apparatus and a magnetic disk apparatus, the tracking servo control or the access control is widely carried out. According to the tracking servo control, an optical pickup and an optical head, which are provided for recording and reproducing of information with respect to a disk medium, are positioned on a specific track of the disk. According to the access control, the optical pickup and the optical head are quickly moved to a target track. Thus, a motor mechanism is adopted for moving the optical pickup and the optical head in the radial direction of the disk.
The conventional optical disk apparatus, as shown in FIGS. 25(a), 25(b) and 26 for example, is arranged so that the optical disk 51 (an information recording medium) is rotated by a spindle motor 53, the spindle motor 53 being provided on a chassis 52. On the lower surface side of the optical disk 51, an optical pickup 54 is provided on the chassis 52 so as to move in the radial direction of the optical disk 51 (later described). The optical pickup 54 carries out recording and reproducing of information with respect to a track 51a on the optical disk 51 while projecting a laser beam 54a which is focused by an objective lens 55.
On the other hand, there is provided on the chassis 52 a permanent magnet 58, guide rails 59 and other members. Such an arrangement constitute a so-called motor mechanism, which is referred to as a linear motor 60, for moving the optical pickup 54 along the guide rails 59, i.e., in the radial direction of the optical disk 51 in accordance with a driving force which is generated by (1) a magnetic field which varies depending on a current applied to a driving coil 57 and (2) a magnetic field due to the permanent magnet 58.
There is provided an FPC (Flexible Printed Circuit) substrate 61 for carrying out inputting and outputting of a power supply current, a detected signal, and a control signal between the optical pickup 54 or the driving coil 57 and a control circuit (not shown). The members, such as the optical pickup 54, the bearings 56 and the driving coil 57, which are moved relatively to the chassis 52 are hereinafter referred to as a motor movable section 60a of the linear motor 60. In contrast, the members, such as the permanent magnet 58 and the guide rails 59, which are not moved relatively to the chassis 52 are hereinafter referred to as a motor stationary section 60b of the linear motor 60.
By the way, in the tracking servo control of the optical disk used as a computer memory apparatus, it is necessary to make the laser beam 54a follow up the track 51a, with an accuracy of not more than 0.1 micron, which displaces by a maximum of several tens of micro meters in accordance with the rotation of the optical disk 51.
Accordingly, the two-stage servo system is widely adopted in which the high accurate tracking servo control is carried out. According to the first stage of the two-stage servo system, the tracking is carried out by the fact that the optical pickup 54 is entirely moved by the linear motor 60 for components, having low frequencies and great amplitudes, of the displacement of the track 51a. According to the second stage of the two-stage servo system, the tracking is carried out by the fact that the objective lens 55 which converges the laser beam 54a is moved by a mechanism referred to as a lens actuator 54b inside the optical pickup 54 for components, having high frequencies and small amplitudes, of the displacement of the track 51a.
Therefore, in such a two-stage servo system, the optical pickup 54 is therein provided with a TES detecting circuit 54c which detects a position gap between the laser beam 54a and the track 51a on the optical disk 51 and outputs the detecting signal as a tracking error signal TES.
Moreover, there are provided a first phase compensating circuit 62 and a second phase compensating circuit 63 for respectively carrying out, with respect to the inputted tracking error signal TES, the appropriate amplifying and the appropriate giving of frequency characteristics. There are further provided a first driver 64 and a second driver 65 for respectively generating a driving current for the lens actuator 54b and for the linear motor 60 in response to the output signals from the respective first and second phase compensating circuits 62 and 63.
FIG. 27, for instance, shows an example of the servo gain curve of such a two-stage servo system. The lens actuator 54b is often arranged so that the objective lens 55 is supported by a spring. Such a case shows the characteristic of a spring-mass system having a resonance point at a frequency f.sub.A is shown.
Moreover, in order to ensure the stability of the servo system in the vicinity of the cutoff frequency fc at which the servo gain becomes 1, a phase lead compensation is carried out for the tracking error signal TES inputted in the frequency range from f.sub.3 to f.sub.4 by the first phase compensating circuit 62 of FIG. 26.
Since the linear motor 60 is arranged so that the optical pickup 54 is supported by bearings 56 so as to be movable therein, the linear motor 60 does not show the resonance characteristic in principle but shows the second-order integral characteristic of the inertial mass system.
It is difficult to follow up the high frequency components of the displacement of the track 51a since the size of the motor movable section 60a is greater compared with that of the lens actuator 54b. Moreover, unnecessary higher-order resonance modes are easy to occur since the size of the motor movable section 60a is greater compared with that of the lens actuator 54b. In order to meet the facts, the second phase compensating circuit 63 as shown in FIG. 26 is provided with a low pass filter which has a corner frequency at f.sub.LPF.
Therefore, an affection dug to the foregoing unnecessary higher-order resonance modes at a frequency f.sub.D can be avoided. There is further provided a first-order low pass filter which has a corner (or pole) frequency at f.sub.1 to improve the servo gain of the linear motor system in the low frequency range.
It is clear from this servo gain curve that the gain of the linear motor system is greater than that of the lens actuator system for the frequencies lower f.sub.X, f.sub.X indicating an intersection frequency of the gain curve of the linear motor system and that of the lens actuator system. So, the linear motor 60 predominantly carries out the servo tracking for the frequencies lower than f.sub.X. Since f.sub.X is set to several hundreds of Hz and the rotational frequency of the optical disk 51 is several tens of Hz, the linear motor 60 predominantly carries out the servo tracking for the low frequency components of the track 51a.
The most important reason why such a two-stage servo system is used lies in the fact that a demerit of the push-pull method, which is widely used for tracking error signal detection in the optical disk apparatuses, is relieved.
For example, as described at page 87 to 88 in "Hikari Disk Gijutsu" (Optical Disk Technology) issued by "Radio-Gijutsusha", in the case where the push-pull method is used for the tracking error detection, the following problem arises. More specifically, when the displacement of the lens actuator 54b is great, the tracking error signal does not become zero even if the position of the laser beam 54a coincides with that of the track 51a. Namely, a tracking offset due to a beam shift occurs, so the accurate tracking servo control cannot be carried out.
However, in the two-stage servo system, since the linear motor 60 follows up the displacement, having components of great amplitude and low frequency, of the track 51, the lens actuator 54b only has to follow up the displacement, having components of small amplitude and high frequency, of the track 51a. Thus, the demerit of the push-pull method is greatly relieved.
In the two-stage servo system, as already described, the linear motor 60 and the lens actuator 54b follow up the displacement of the track 51a in cooperation. This two-stage servo system does not work well unless both the characteristics of the linear motor 60 and that of the lens actuator 54b are in good conditions.
Since the lens actuator 54b is composed of compact and lightweight elements, the stiffness and accuracy of each element or the assembly accuracy is relatively high. Therefore, the characteristic distortion, due to undesired resonances at the foregoing elements' side, presents no serious problem. This is because even if such resonances occur, the resonance frequencies are likely to be enough higher than the frequency bandwidth (normally several kHz) required for the servo control.
In contrast, the linear motor 60 has a great size and heavy weight, various characteristic distortions may occur for the frequencies of not higher than several hundreds of Hz. Therefore, it is an important problem to keep the characteristics appropriate.
The characteristic distortions are likely to occur not only due to the higher-order resonance modes (occurring at frequency f.sub.D), as shown in FIG. 27, but also due to the spring characteristic or non-linearity of the bearings 56 and the FPC substrate 61 which is provided for carrying out of the input and output of the power and the signals with respect to the optical pickup 54.
FIG. 28 shows an example of the transfer characteristic of the linear motor 60 by which the displacement of the motor movable section 60a varies depending on the applied driving current. When rolling-type bearings, for example, are used in the linear motor 60, it sometimes occurs that the bearings have an equivalent spring characteristic due to minute elastic deformation in a rolling contact surface. Thus, the resonance characteristic of the spring-mass system, due to this spring characteristic and the mass of the motor movable section 60a, is sometimes shown like the broken line of FIG. 28.
If there is not such a spring characteristic, the characteristic of the inertial mass system having simply a gain curve slope of -40dB/dec ought to be shown as the solid line of FIG. 28. However, in the case where the above-described resonance characteristic of the spring-mass system is shown, the response (that is, a servo gain) of the linear motor 60 for the frequencies lower than its resonance frequency f.sub.L is reduced. Such a spring characteristic also occurs due to the bending of the FPC substrate 61, and similarly the servo gain is reducued.
Ordinarily, the spring characteristic of the bearings 56 or the FPC substrate 61 is weak. So, even if the resonance occurs, the frequency f.sub.L due to the resonance is so low that there presents no problem. However, in the case where, for example, the pressure for the bearings 56 is increased in order to prevent the mechanic looseness of the bearings 56, f.sub.L becomes high, and the linear motor 60 is not concerned with the servo control any longer for the frequencies lower than f.sub.L.
Moreover, sliding-type bearings made of fluorine resin (fluorine-contained polymers) or so are sometimes used as the bearings 56 instead of the rolling-type bearings in order to lower the size of the apparatus. In this case, the equivalent spring characteristic of the bearings 56 is easy to be greatly changed, i.e., the non-linearity of the spring characteristic is easy to occur remarkablly, due to the environmental temperature and the humidity or the displacement amount.
FIG. 29 shows an example of a servo gain curve of the two-stage servo system wherein the above-described sliding-type bearings 56 are used in the linear motor 60. The resonance frequency due to the mass of the motor movable section 60a and the equivalent spring characteristic of the bearings 56 is located at an enough low f.sub.L when the displacement of the track 51a is great. However, the resonance frequency becomes high up to f.sub.L ' when the displacement of the trace 51a is small, and the servo gain for the frequencies lower than f.sub.L ' is reduced.
If such a high resonance frequency occurs merely when the displacement is small, there presents no problem since the displacement of the track 51a to be followed up is small even if the servo gain is accordingly reduced.
However, due to the temperature or humidity, the movement of the sliding-type bearings 56 is sometimes made badly, and the bearings 56 are stuck in an extreme cases. In this case, f.sub.L ' becomes extremely high so as to be in order of hundreds of Hz or a few kHz. Then, the servo gain is remarkably reduced regardless of the amount of the displacement of the track 51a.
Alternatively, the response of the linear motor 60 to the driving current, i.e., the sensitivity, changes due to (1) the difference of the magnetic attraction of the permanent magnet and the driving coil which are for giving the driving force to the linear motor 60 or (2) minute shape deformation thereof. The servo gain is affected by the change of such sensitivity. When the sensitivity is low, there presents a problem that the servo gain is so reduced that the following performance for the track 51a is educed. On the other hand, even if the sensitivity is high, there occurs another problem.
FIG. 30 shows an example of a servo gain curve of the two-stage servo system wherein the linear motor 60 whose sensitivity is so high as above described is used. The gain curve changes along the broken line of FIG. 30 in accordance with the improvement in the sensitivity.
At this time, it is likely to occur that the entire servo system including the lens actuator system will become unstable since the affection due to the higher-order resonance modes at f.sub.D in the above-described linear motor 60 becomes great.
This variation of the sensitivity also occurs due to the relative position of the motor movable section 60a to the motor stationary section 60b, the relative position being derived from the non-uniform magnetic flux distribution of the permanent magnet.
Moreover, there presents another problem different from the characteristic distortion and parameter variation. Namely, in the case where some disturbances such as vibrations or shocks are applied to the apparatus, the linear motor 60 in the conventional optical disk apparatus or so is directly affected by them and the accuracy of the servo control is deteriorated.
Conventionally, some studies have been made for the problems that the bad affections occur due to the distortion characteristic including occurrence of the resonance, the variation of the sensitivity, or disturbances such as vibrations and shocks.
For example, in K. Onishi (the Journal of the Institute of Electrical Engineers of Japan vol.110, no.8 (1990), p.657-600), the study which aims controlling of the joint of the robot and intends to solve the above-described problem is reported.
According to the article, the deviation of the motor characteristic and response from the ideal ones, due to the characteristic distortion, the parameter variation, vibrations, and shocks, is all considered to be derived from the fact that the disturbance torque is applied to the motor. Accordingly, a so-called disturbance observer method, wherein the entire estimated disturbance torque is fed back to the motor, is disclosed in that article.
FIG. 31 shows a block diagram of the system using the above-described disturbance observer method, and FIG. 32 shows a block diagram of the system which is re-drawn by an equivalent conversion.
In the disturbance observer method in the system shown in FIG. 32, the right half configuration is for obtaining the entire torque T.sub.1 which is applied to the motor, while the left half configuration is for obtaining a nominal value T.sub.2 of the driving torque. The torque T.sub.1 includes a driving torque T.sub.m and a disturbance torque T.sub.dis, and is obtained by (1) converting the rotation velocity (angular velocity) .omega. which is detected by the sensor into the angular acceleration with approximate differentiation in the band whose upper angular frequency is g, and then (2) multiplying the angular acceleration by the reduced amount g during the approximate differentiation and the nominal value J.sub.n of the inertial moment. The nominal value T.sub.2 is obtained by multipling the driving current I of the motor by a nominal value K.tau.n of the torque constant.
The above-described disturbance observer method obtains the estimated value T.sub.dis ' of the disturbance torque from the difference (T.sub.2 -T.sub.1) and multiplies this difference by a reciprocal of the nominal value K.tau.n of the torque constant so as to obtain a compensating current I.sub.cmp for canceling the above-described difference.
In the system using such a disturbance observer method, the above-described compensating current I.sub.cmp is added to a reference value I.sub.a.sup.ref of the driving current for the motor so as to obtain the actual driving current for the motor, and the actual driving current is added to the motor so as to suppress the affections due to such as the distortion characteristic in the motor, the parameter variation, vibrations and shocks applied from outside.
Additionally, the low pass filter [g/(s+g)] in the left half configuration of the disturbance observer, obtained by the equivalent conversion, has no affection for the frequencies lower than the bandwidth g where the approximate differentiation is executed, and limits a band for the frequencies lower than the bandwidth g.
However, in case where the above-described conventional disturbance observer method is adopted to the motor control device for the information recording and reproducing apparatus, there presents a big problem as follows. Namely, first, as for a sensor for detecting the movement of the motor, a tacho generator detecting a rotation velocity (angular velocity) can be easily used since the above-described study deals with a rotation-type motor.
In order to add a similar sensor detecting the velocity to the linear motor 60 of a linear-moving type, as shown in FIG. 33, for example, the structure where a magnet 66 is attached to the chassis 52 and a detecting coil 67 is attached to the motor movable section 60a is conceivable.
Moreover, as shown in FIG. 34, another structure is conceivable, wherein a photo detector 68 for position detection is attached to the chassis 52 and a light emitting diode 69 is attached to the motor movable section 60a, and wherein a differentiator 70 for outputting a velocity by differentiating the displacement output detected by the photo detector 68.
However, the foregoing respective structures are bulky because of bridging the motor stationary section 60b and the motor movable section 60a. Therefore, the positioning accuracy of the elements installed on the motor stationary section 60b and the motor movable section 60a is very important in order to obtain the predetermined accuracy as a sensor.
Therefore, in the above-described sensor structures, their structures are complicated and the assembly requires much time, thereby presenting the problem that the costs are increased. Moreover, the above-described sensor structures are bulky, thereby causing to invite the danger of generating another unnecessary resonance due to the assembly.
FIG. 35 shows an example of a system using a disturbance observer in which a velocity sensor is used and the controlling is carried out with respect to a linear motor instead of a rotation-type (or rotary) motor. Variables or so are rewritten in FIG. 35 according to the linear motor 60. Additionally, a sensor required for detecting the velocity is illustrated by a block as Cs in FIG. 35, and the meanings of the variables or so in the figure are as follows.
K.sub.f :driving force (thrust) constant of the linear motor
M :mass of the linear motor movable section
K.sub.fn :nominal (reference) value of K.sub.f
M.sub.n :nominal (reference) value of M
C :sensitivity of the velocity sensor
F.sub.m :driving force (thrust) of the linear motor
F.sub.dis :disturbance force (including parameter variations or so)
F.sub.dis ' :estimated value of the disturbance force (including parameter variations or so)
I.sub.a.sup.ref :reference value of the driving current to the linear motor
I :actual driving current for the linear motor
I.sub.cmp :current for compensating the disturbance force
x :displacement of the linear motor
s :Laplace operator
g :estimated band of disturbance of the observer (angular frequency)
There is a block having amplification degree of (g.multidot.M.sub.n /C) as a gain term in the above-described system. As the bandwidth g of the approximate differentiator becomes greater, the ]]eater amplitude is required for the above-described block of (g.multidot.M.sub.n /C).
For example, in order to obtain a good characteristic up to about 2 kHz in a linear motor of optical disk apparatuses, g=2.pi..times.2000.apprxeq.12600 is satisfied. However, it is not easy to realize an amplifier having such a high gain. Even if an OP (operational) amplifier is used as the above-described amplifier, the amplifier must have the frequency bands of not less than about 25 MHz, or the amplifier must be composed of a plurality of amplifiers in series.
Therefore, such amplifiers having wide bands are expensive, and such plural amplifiers are expensive since their configuration becomes complicated. Moreover, in the above-described structure having such a high gain, the following inconvenience occurs that the noises generated by the amplifier itself and externally applied noises are amplified in accordance with the amplification degree.
If (M.sub.n /C) is reasonably small, the block of (g.multidot.M.sub.n /C) can be composed of an amplifier having a low gain. However, in a linear motor of the optical disk apparatus, the mass M of the motor movable section 60a, which is ordinarily not so far from M.sub.n, is about 0.01 kg at lightest, and the sensitivity of the velocity sensor is in the order of 0.01V/ms.sup.-1 at most. Taking the fact into consideration, it cannot be expected that (g.multidot.M.sub.n /C) becomes smaller than g.
Moreover, the approximate defferentiation [s/(s+g)] to the bandwidth g is carried out when estimating the disturbance torque based upon the detected velocity information. However, the differentiation emphasizes respective noises from the sensor, from the circuit, and from outside, so the motor control based upon the fact causes waste of electric power and heating of the motor itself, and causes unnecessary vibrations and noises in the motor.
The characteristic distortion, the parameter variation, vibrations, and shocks can be absorbed and compensated up to the bandwidth g where the defferentiation is carried out. But if the bandwidth g is expanded, the influence of the noises becomes great accordingly.
As shown in FIG. 27, for the linear motor 60 in the optical disk apparatus, good characteristic is necessary up to the frequency band of three or four times as much as f.sub.X, which is a frequency at the intersection of both the gain curve of the linear motor system and that of the lens actuator system. For example, when f.sub.X is 500 Hz, it is desirable to suppress the occurrence of the characteristic distortion and to absorb the bad affection of the characteristic distortion or so up to about 2 kHz. However, it is hard to realize the peripheral circuits which include a velocity sensor or a differentiator and have few noises up to such a frequency band.
Moreover, considering that a block having a great gain of (M.sub.n /C) as above-described is necessary, the above-described problem of noises of the velocity sensor or the differentiator becomes more serious.